5T4 (also designated trophoblast glycoprotein, TPBG, M6P1 and Waif1) is a well-defined tumor-associated antigen (TAA) originally identified by Professor Peter Stern, University of Manchester (Hole and Stern, 1988). It is an oncofetal antigen expressed in a high proportion of patients in a variety of malignancies, including non-small cell lung, renal, pancreas, prostate, breast, colorectal, gastric, ovarian and cervix cancers as well as in acute lymphocytic leukemia, and has also been shown to be expressed in tumor-initiating cells (Castro et al., 2012; Damelin et al., 2011; Elkord et al., 2009; Southall et al., 1990).
Although low levels of 5T4 expression have been detected in some healthy tissue, such as the placenta and specialized epithelia, expression levels in tumors are considerably higher.
Data suggest that 5T4 regulates the functional activity of CXCR4 (Castro et al., 2012; Southgate et al., 2010). 5T4 binding antibodies or 5T4 knock-down resulted in inhibition of CXCR4-mediated cellular migration, a pathway involved in tumor growth and metastasis. Therefore, targeting 5T4 may provide therapeutic benefits in the treatment of various cancers. Currently there are no FDA approved therapeutics that specifically target or bind 5T4. There is a need for new therapeutics to treat malignancies in which 5T4 is expressed.
4-1BB (also known as CD137 or TNFRSF9) is a tumor necrosis factor (TNF) receptor (TNFR) superfamily member. 4-1BB is expressed on various cell populations including activated CD4+ and CD8+ T cells, regulatory T cells (Treg), dendritic cells (DC), monocytes, mast cells, eosinophils and tumor endothelial cells. Activation of 4-1BB is dependent on receptor oligomerization (Rabu et al., 2005; Wyzgol et al., 2009) induced by binding to 4-1BBL (also known as CD137L), which is expressed as a trimer on the cell surface of antigen presenting cells (APCs) and other cell types. 4-1BB activation on CD8+ T cells sustains and augments CD8+ T cell effector functions and preferentially supports Th1 cytokine production (Shuford et al., 1997; Lee et al., 2002; Pulle et al., 2006). 4-1BB activation on CD4+ T cells, 4-1BB stimulation initially results in activation and later in activation-induced cell death, which is thought to explain why 4-1BB agonistic antibodies have shown therapeutic effect in tumor immunity as well as in autoimmunity (Zhang, J C I, 2007; Sun, Trends Mol Med, 2003). 4-1BB activation has also been reported to suppress Treg function or convert Tregs to cytotoxic CD4+ T-cells (Akhmetzyanova et al., 2016; So et al., 2008).
In addition to expression on and modulation of T cell effector function, 4-1BB is upregulated on CD16- and cytokine-activated natural killer (NK) cells. Activation of 4-1BB has been shown to increase antibody-dependent cellular cytotoxicity (ADCC) activity of NK cells in both murine and human cells (Kohrt 2012 and 2014 J Clin Invest, reviewed by Hout 2012, Oncoimm). Further, activation of 4-1BB expressed on APCs, such as DCs and macrophages may also induce and/or modulate immune activation.
The role of 4-1BB in the modulation of immune cell activation suggests that it may be a desirable immunotherapy target in the treatment of multiple cancer types. Indeed, two 4-1BB antibodies are in clinical development: Urelumab (BMS-66513) developed by Bristol-Myers Squibb and PF-05082566 developed by Pfizer. Phase I and II studies in various indications are ongoing for each of the antibodies. However, liver and skin toxicities have been observed in patients and murine models upon 4-1BB activation (Ascierto et al., 2010; Dubrot et al., 2010; Niu et al., 2007). Further, a Phase II study with Urelumab as a second line therapy in metastatic melanoma was terminated in 2009 due to liver toxicity (Garber et al., 2011; Li and Liu, 2013).
Therefore, there remains a need in the art for therapeutics that safely and effectively activate 4-1BB for use in the treatment of oncologic indications.